1. Field of the Invention
The present invention relates in general to a grinding wheel having a center mounting hole at which the grinding wheel is to be mounted on a wheel spindle, and more particularly to such a grinding wheel having a bushing member fitted in an inner circumferential surface of the center mounting hole.
2. Discussion of the Related Art
There is known a generally disc- or cup-shaped grinding wheel having a mounting hole formed at a center thereof about which the grinding wheel is to be rotated. A wheel spindle of a grinding machine or a mounting member attached to the wheel spindle is fitted into the mounting hole, so that the grinding wheel is rotatable together with the wheel spindle. The wheel spindle or the mounting member, which is fitted in the mounting hole, tends to be worn due to its contact with abrasive grains which cooperate with a vitrified bond or an organic bond to constitute a vitrified abrasive structure or a resinoid abrasive structure of the grinding wheel. The wearing of the wheel spindle or mounting member causes deterioration of a coaxial or concentric relationship between the wheel spindle and the grinding wheel or between the mounting member and the grinding wheel, accordingly resulting in deteriorated machining accuracy of the grinding wheel.
There is also known a grinding wheel having a bushing member bonded to an inner circumferential surface of a central mounting hole formed therein. The bushing member consists of a layer of lead, sulfur or carbovidro bond which is prepared by adding graphite to sulfur. In the process of forming the bushing member, the lead, sulfur or carbovidro bond is first heated into a molten state, and the molten lead, sulfur or carbovidro is then poured into a suitable mold so that the molten material is hardened in the mold. The bushing member may consist of a layer of other material, such as a layer of gypsum which is prepared by pouring a slurry of gypsum into a mold, as disclosed in JP-U-06-83258. Such a grinding wheel having the bushing member provides an advantage that the wheel spindle or mounting member is prevented from being worn since the wheel spindle or mounting member is introduced into the no bushing member so as to be fitted in the bushing member without contact of the wheel spindle or mounting member with the abrasive grains.
However, the above-described bushing member does not have a sufficiently high strength, and is not bonded to a main body of the grinding wheel with a sufficiently high bonding strength. In addition, the bushing member made of lead cannot be easily formed due to rapid solidification of molten lead, particularly, where the main body of the grinding wheel has a relatively large outside diameter and the mounting hole has an accordingly large inside diameter, or where the main body of the grinding wheel has a relatively large axial length and the mounting hole has an accordingly large axial length. The bushing member made of sulfur or carbovidro bond suffers from a relatively large degree of chronological deterioration or aging, and does not have a sufficiently high degree of bonding strength with which the bushing member is bonded to the main body, resulting in a risk of cracking or separation of the bushing member from the main body, depending upon a condition in which the grinding wheel is stored. The bonding strength is further reduced, for example, where the abrasive grains have a grain size not larger than #240. The bushing member made of gypsum is difficult to be formed due to a relatively long time required for hardening the gypsum, and does not have a high degree of strength, resulting in a large risk of being cracked during a grinding operation.
It is therefore an object of the present invention to provide a grinding wheel having a bushing member which can be easily formed even where a mounting hole of the grinding wheel has a large diameter or a large axial length, and which has high degrees of durability and strength with a reduced degree of chronological deterioration and exhibits a high degree of bonding strength with respect to a main body of the grinding wheel.
Various studies made by the present inventors revealed that it is possible to easily form a bushing member by using an urethane resin as a material for the bushing member, and that the bushing member formed of the urethane resin has sufficiently high degrees of durability and strength and provides a sufficiently high degree of bonding strength with respect to the main body of the to grinding wheel.
Therefore, the above-described object may be achieved according to the principle of the present invention, which provides a grinding wheel having a main body and a tubular bushing member which is fitted in an inner circumferential surface of a mounting hole formed through the main body, so that the grinding wheel is mounted on a wheel spindle of a grinding machine, with the bushing member being fitted on the wheel spindle, wherein the bushing member includes an urethane resin as a main component thereof.
The urethane resin, which is used as the main or major component of the busing member of the present grinding wheel, has a relatively low viscosity before hardening thereof, so that the urethane resin is easily poured into a cavity of a mold which defines a desired shape of the bushing member, and permeates or enters into small cavities and pores of abrasive structure of the main body of the grinding wheel. The time required for hardening or curing the urethane resin is about two minutes, which is as short as that for hardening a sulfur. Thus, the bushing member is formed with a simple operation, even where the mounting hole has a large diameter and/or a large axial length, namely even where the bushing member is required to have a large diameter and/or a large axial length. The bushing member constituted by the hardened urethane resin has a high degree of durability with reduced chronological deterioration or aging, and a high degree of strength with which the bushing member is bonded to the main body.
According to a first preferred form of the grinding wheel of the invention, the urethane resin has a viscosity of 15-200 mPaxc2x7s (=15-200 c.p.s) before hardening thereof. Owing to the low viscosity, the urethane resin can be easily poured into the cavity of the mold, and the poured urethane resin can be distributed evenly in the entirety of the cavity of the mold and enters into the small cavities and pores of the abrasive structure of the main body of the grinding wheel, resulting in a further increased bonding strength.
According to a second preferred form of the grinding wheel of the invention, the urethane resin has a bending strength of 10-40 MPa and an elastic modulus of 8-25 GPa after hardening thereof. Such material characteristics of the urethane resin provides the bushing member with a high degree of impact or shock resistance and a suitable degree of strength, so that grinding wheel can be mounted on the wheel spindle of a grinding machine with high precision, and so that an impact or shock applied to the bushing member is absorbed by the bushing member, protecting the main body of the grinding wheel from the impact or shock.
According to a third preferred form of the grinding wheel of the invention, the urethane resin has a hardness ranging from D-50 to D-70 after hardening thereof. This hardness of the urethane resin makes it possible to smoothly introduce the wheel spindle into the bushing member, and minimize wear of the wheel spindle which is fitted into the bushing member.
According to a fourth preferred form of the grinding wheel of the invention, the main body consists of an abrasive structure in which abrasive grains are held together by a vitrified bond.
According to fifth preferred form of the grinding wheel of the invention, the bushing member has a wall thickness of 0.2-2.0 mm as measured in a radial direction thereof.